Broken spark plug

I have a 98 gmc sierra 1500. About every 2500-5000 miles the porcelain on the#4 spark plug breaks inside the cylinder. It’s been doing this for years and my mechanic is at a loss as to what is causing it Any ideas?

You might have some carbon build up which could lead to hot spots and “pre-ignition”. You should hear some knocking or “pinging” due to pre-ignition but you might be missing it. Pre ignition could break a plug ceramic. You might try higher octane gas.

I would suspect a clogged EGR passage to that cylinder causing spark knock.

What brand spark plug? Not all plugs penetrate the same distance into the cylinder. I’ve compared depths of some plugs to others. One of the reasons I prefer Champions or Boschs as they tend to be the correct length.

Following Uncle Turbo and Circuitsmith’s suggestion, you may need to clean out your throttle valve also. Easy one to diagnose as your pedal seems to stick a bit (not engine choke or stall, the pedal actually seems sticky).

Check all hoses leading to your EGR, or if you haven’t ever, replace all of the vacuum hoses to and from the EGR. Before hooking up the hoses, check for PCM trouble codes.

1.EGR Valve Fault Codes that should light up your CHECK ENGINE LIGHT.

2.EGR Valve Fault Codes:
◦P0401 EGR System Flow Insufficient.
◦P0403 EGR Solenoid Circuit.
◦P0404 EGR System Performance.
◦P0405 EGR Pintle Position Circuit Low Voltage.
◦P1404 EGR Valve Stuck Open.
◦P1406 EGR Valve Pintle Position.

3.Rough Idle, choking idle.

4.gas mileage has gotten worse, especially for start and go traffic.

5.Lack of power as you accelerate the vehicle down the road. Careful about this idea, as the big GMC engines can still kick in with a lot of power by pushing further on the gas pedal and pouring gas into the engine. Think of it as the engine sounds tired as you accelerate from a stop.

6.Engines seems to run ok above 30 miles and hour but once you come to an idle, the engine barely stays running.

When something goes wrong with the EGR System… look for:
1.Carbon blocking the EGR Valve’s Pintle from sealing.
When a piece of carbon blocks the EGR Pintle, you get one of the following Diagnostic
Trouble Codes: P1404 or P1406.

2.The EGR passages inside the Intake Manifold get clogged with carbon. •When this happens, you’ll get a Diagnostic Trouble Codes P0401.

More at including some specifics for your engine.

Also, before buying any ‘cleaning sprays’ at the local auto store, check the manual for what kind of sprays are allowed. The most common carb and throttle body cleaners have solvents that quickly weaken or even strip plastic insulation or bushings. Especially that can of carb cleaner that’s been in the garage ‘just in case’ for decades.